EARTHQUAKE SOURCE PARAMETER AND FOCAL MECHANISM ESTIMATES FOR THE WESTERN QUEBEC SEISMIC ZONE IN EASTERN CANADA

The Western Quebec Seismic Zone (WQSZ) is a 160-km-wide band of intraplate seismicity extending 500 km from the Adirondack Highlands (United States) to the Laurentian uplands (Canada). Historically, the WQSZ has experienced over fifteen earthquakes above magnitude 5, with the noteworthy M_N 5.2 Ladysmith event on May 17, 2013. The relationship of the WQSZ to pre-existing geological structures is not clearly established. Previous studies have associated seismicity in the area to the reactivation of Early Paleozoic normal faults within a failed Iapetan rift arm, or strength contrasts between mafic intrusions and felsic rocks due to the Mesozoic track of the Great Meteor hotspot. A good understanding of seismicity and its relation to pre-existing structures requires information about event source properties, such as static stress drop and fault plane orientation, which can be constrained via spectral analysis and focal mechanism solutions. In this study, we characterize the b-value, conduct a systematic spectral analysis to derive corner frequency, seismic moment, and static stress drops, and analyze first arrival polarities to derive focal mechanism solutions of selected events.

Using data recorded by the CNSN and USArray Transportable Array, we first characterize b-value for 709 events between 2012 and 2016, obtaining a value of ~ 0.75. We then determine corner frequency and seismic moment values by fitting S-wave spectra on transverse components at all stations for 35 events M_N 2.7+. We use the corner frequency and moment estimates to calculate moment magnitude, static stress drop values, and rupture radii, assuming a circular rupture model. Our preliminary results show corner frequencies of ~ 15 to 40 Hz and stress drop values between ~ 7 and 130 MPa, typical of intraplate seismicity, higher than those for interplate seismicity. Last, we solve focal mechanism solutions of 35 events with impulsive P-wave arrivals at a minimum of 8 stations using the hybridMT moment tensor inversion algorithm. Our preliminary results suggest predominantly thrust faulting, and at times oblique thrust faulting. The P-axis trend of the focal mechanism solutions suggests a principal stress orientation of NE-SW, which is consistent with that derived from focal mechanisms of earthquakes prior to 2013.